+1, the freehub or freewheel ratchet may be a bit sticky or gummy adding some extra drag. This isn't rare with new hubs or wheels, since the lube can dry slightly on the shelf. The added friction causes the freewheel to drag, transferring slack from the lower to the upper loop when you backpedal.

The cure is usually a matter of adding 2 drops of oil to the freehub to soften dried grease. You can also ride a while and the problem may resolve over a few days as the internals loosen up with use.

There are also other causes of freehub drag, so if oil doesn't help, have someone take a look at at.